1. Field of the Invention
The present invention relates to an antenna structure for a mobile phone, and especially to an antenna structure for a mobile phone of which metal is used to obscure around a base board of the antenna to improve the hearing aid compatibility (HAC) character of the antenna structure.
2. Description of the Prior Art
A standardization corporation ANSI (American National Standards Institute) in the U.S.A. stipulated a specification of ANSI C63.19, the FCC (Federal Communication Commission) asked the manufacturers and service agents of mobile phones for the necessity of over 50% satisfaction in meeting the regulation of the limitation of EMI for hearing aids stipulated in ANSI C63.19 on products input to the U.S.A. since Feb. 18, 2008.
The standard in testing stipulated by the specification on the hearing aid compatibility (HAC) in ANSI C63.19 is as below:    a. to use a testing probe for measuring the quantity of the electromagnetic field of 5×5 cm2 and 15 mm above a sound outputting hole of a mobile phone;    b. to divide the tested plane into 9 blocks, each block is separately tested on its electromagnetic field strength;    c. to omit the electromagnetic field strengths of the largest three blocks among the 9 blocks, and to take the block with the largest electromagnetic field strength among the remaining six blocks to define the category of HAC;    d. different categories of HAC are defined by having a difference of value of 5 dB between every two categories, and include the categories of M1, M2, M3, M4 etc. (in which M3 and M4 are categories meeting the standard of stipulation).
Therefore, normally people will observe the HAC categories of electric field and the magnetic field at the same time of an antenna structure, and also omit the field strengths of the largest three blocks, but to take the block with the largest electromagnetic field strength among the remaining six blocks to define the HAC at the present frequency.
In order to meet the specification for the antenna structure under the condition not to lower efficiency, the present invention provides a brand new measure of solution to elevate the category of the antenna structure.